In high resolution synthetic aperture radar (SAR) systems, it is critical to know the exact position (to within a small fraction of the carrier wave length) of the antenna during integration time in order to produce images of acceptable quality. This problem has been aptly described in U.S. Pat. No. 4,134,113 issued to Norman F. Powell on Jan. 9, 1979, and wherein such document is pertinent to the present invention.
Briefly, and referring now to FIG. 1 in the drawings, a SAR system carried by an aircraft AC maps a target region T by transmitting and receiving radar signals at various sampling points S along the flight path FP of the aircraft. The SAR system may be positioned in the nose portion NP of the aircraft AC, for example. The SAR will typically require an inertial reference system INS for calculating the antenna AN position at the sampling points S. A problem with high resolution SAR systems is that the antenna AN may be physically separated from the inertial navigation system INS by a physical support, such as a lever arm LA, for example. The flexible modes of the lever arm render imprecise the data indicating the position of the antenna. What this means is that the antenna AN may deviate from its position as reported by the inertial navigation system INS. In other words, and referring now to FIG. 9, at every sampling point S in the flight path FP, the antenna AN may be displaced slightly to a position denotes S'. This may be referred to as antenna residual motion.
Residual motion of the antenna AN may cause corruption of the reflected radar signal received from the target region T. If such residual motion is not compensated for in some manner, the image produced by the SAR may be degraded to a point such that it would be of no practical use. A person skilled in the art would, of course, realize that the severity of antenna residual motion becomes a greater problem with increases in the desired resolution of the SAR.
The present invention provides a method for compensating for antenna residual motion in a SAR system.